Zone refining method with plural supply rods



Nov. 10, 1970 w. KELLER 3,539,305

ZONE REFIXING METHOD WITH PLURAL SUPPLY RODS Filed Sept. 27, 1967 United States Patent US. Cl. 23-301 12 Claims ABSTRACT OF THE DISCLOSURE Method of crucible-free zone melting a crystalline rod which comprises rotating at least one of a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, heating the rod with an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod dividing the rod into a supply rod portion being supplied to the melt in the molten zone and a rod por tion resolidifying from the melt, relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed and laterally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder so that the resolidifying rod portion is increased in thickness beyond the inner diameter of the annular heating device, and thereafter supplying at least one additional supply rod to the melt at a location thereof eccentric to the axis of the resolidifying rod portion, preheating the end of the additional supply rod to melting temperature in the vicinity of the molten zone to bringing that end and the melt into mutual engagement.

My invention relates to method of crucible free zone melting a monocrystalline rod, especially a semiconductor rod.

In my copending application Ser. No. 428,933, filed Jan. 29, 1965, now US. Pat. 3,414,388 I disclose method for crucible-free zone melting a crystalline rod which comprises rotating at least one of a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, and heating the rod With an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod, dividing the rod into a supply rod portion being supplied to the melt in the molten zone and a rod portion resolidifying from the melt. My method further comprises relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed and laterally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder so that the resolidifying rod portion is increased in thickness beyond the inner diameter of the annular heating device. The rod portion resolidifying from the melting zone can be located selectively below or above the heating device.

The method of my aforementioned copending app-lication is carried out as follows:

At the start, both holders of a crystalline rod are disposed along the same vertical axis. Thereafter, the holder of the rod portion resolidifying from the melting zone is displaced relative to the holder of the rod portion to be melted and to the heating device in a progressively lateral direction and in an upper direction. The cross section of the rod portion resolidifying from the melting zone is thereby continually increased. As soon as the nominal 3,539,305 Patented Nov. 10, 1970 cross section of the resolidifying rod portion is attained, the holder thereof is held fast in the eccentric position thereof and is only thereafter adjusted in elevation. By this method, semiconductor rods, advantageously silicon rods, having larger cross section than that obtainable with the known concentric or coaxial zone-melting methods, wherein both supply rod portion and resolidifying rod portion are always maintained in coaxial alignment are able to be produced with improved crystal quality and comparatively uniform specific resistance distribution over the rod cross section.

In a further development of and improvement over the aforementioned method of my copending application Ser. No. 428,933, now US. Pat. 3,414,388, I have proposed in my copending application Ser. No. 564,118, filed July 11, 1966, now US. Pat. 3,477,811 to laterally displace the resolidifying rod portion in the opposite direction from that in which the supply rod portion is laterally displaced and to repeat the lateral displacement several times during the zone melting operation in both directions. By means of this additional step of the method, the specific resistance distribution over the rod cross section is made even more uniform.

In accordance with another proposal in my application Ser. No. 597,340 filed on Nov. 28, 1966, the semiconductor rod is end-supported in two holders which are displaceable laterally along axes parallel to one another, one of the holders being secured to a vertical shaft. The shaft is mounted eccentrically in a cylinder located in a mounting block and rotatable about a vertical axis. The cylinder is rotated by means of a Worm drive. With this apparatus, the lower rod holder is displaced eccentrically.

It is accordingly an object of the present invention to improve the method described in my copending application Ser. No. 428,933, now US. Pat. 3,414,388, so that the advantages attainable thereby can also be imparted to monocrystalline rods having extremely large radial dimensions, for example having a diameter of 50 mm. and more. It is a further object of the method of my invention to permit the doping, particularly, of such thick rods over the cross section thereof to be maintained as uniformly as possible.

With the foregoing and other objects in view, I provide, in accordance with my invention, method for crucible-free zone melting of the aforementioned type wherein, after terminating the step of laterally displacing the rod portion or portions, one or more additional rods are supplied to the melting zone eccentrically to the axis of the resolidifying rod portion, the end of the additional rod or rods supplied to the melting zone being heated to melting temperature in the vicinity of the melting zone before being mutually engaged with the melt in the melting zone. The rod portions supplied to the melting zone can be heated with separate or with the same heating device. When an inductive type of heating device, such as an induction heating coil, is employed, it is particularly advantageous to perform the heating by means of a single heating coil. For example, when providing two supply rods to the resolidifying rod portion, the induction heating coil can have the form of a figure eight lying on its side. With such a coil arrangement, the melting zone is heated not only from the outside but also from the inside so that a very uniform temperature distribution is provided within the melting zone. The resolidfying rod portion crystallizes out with negligible thermal stresses. Thereby, the frequency of crystal disturbances or dislocations is greatly reduced. Since the diffusion characteristic or capability of the dopant is highly temperature-dependent, uniform doping in the resolidifying rod portion thereby requires the most uniform temperature distribution possible within the melting zone. This requirement is net to a great extent by the method of the invention of the instant application.

In accordance with further features of my invention, after the additional supply rods have been brought into engagement with the melting zone, the resolidifying rod portion is laterally displaced several times in opposite directions. Thereby, the zone of largely constant thermal capacity in the center of the melting zone has a wideranging eifect in the interior of the melting zone between the limits of the lateral displacement. This advantage is also obtained if, after mutually engaging the additional rod or rods and the melting zone, the resolidifying rod portion is placed in rotation about an axis eccentric and parallel to the center axis thereof. This last-mentioned method step is particularly advantageous if all of the rods supplied to the melting zone are simultaneously set in rotation. The melting zone is thereby well intermixed, producing a particularly uniform specific resistance distribution over the rod cross section. It is furthermore advantageous and in accordance with another feature of the method of my invention, after completion of the step of laterally displacing the resolidifying rod portion, to periodically reciprocate the rod supplied to the melting zone about the center location thereof or to subject it to an eccentric motion about its center position.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method for crucible-free zone melting a crystalline rod, especially a semiconductor rod, it is nevertheless not intended to be limited to the details shown, since various ,modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of diiferent modes thereof when read in connection with the accompanying drawings, in which:

FIGS. 1 to 4 are diagrammatic views of the rods and heating device employed in the method of my invention, illustrated in different phases of the method;

FIG. is a top plan view of an induction heating coil for carrying out the method; and

FIG. 6 is a diagrammatic view of the rods and heating device illustrating another mode of the method of my invention.

Referring now to the drawings and first particularly to FIG. 1 thereof there is shown a crystalline rod 1, particularly a semiconductor rod, at the lower end of which a molten zone 3 is produced by means of a heating device 2 preferably an induction coil energized with high-frequency current, to which a monocrystalline seed crystal 4 is fused. The crystalline rod 1 can be polycrystalline. The seed crystal 4 is rotated about its vertical axis, and the semiconductor rod is also rotated about its rotatable axis but preferably in a direction opposite to the rotary direction of the seed crystal 4. One or more additional rods 5, preferably polycrystalline semiconductor rods, are disposed eccentrically to the vertical axis of the semiconductor rod 1 within the same or a separate heating device 2. The heating device 2 can be stationarily mounted in a zone heating chamber which is evacuated or filled with a protective gas atmosphere such as argon, that is not illustrated in the figures.

Further steps of the method are indicated in FIG. 2 by the arrows. The seed crystal 4 is not only moved downwardly with respect to the stationary heating device 2 but also is moved simultaneously sidewise for example toward the left-hand side of FIG. 2, as shown, so that the lower part of the melting zone 3 is drawn toward the left-hand side of FIG. 2. Since the seed crystal 4 rotates about its center axis, the rod portion 6 crystallizing out of the melt grows substantially symmetrically to the ro- 4 tary axis of the lower rod holder 9. The upper rod portion 1 supported at its free end by the rod holder 10 is displaced downwardly at a corresponding rate.

As soon as the lateral displacement operation of the seed crystal 4 is completed, as shown in FIG. 3, the additional rod or rods 5, the end or ends of which are brought to the heating temperature by the heating device 2, are supplied to the melting zone 3. The additional rod or rods 5 are then set in rotation, preferably in an opposite rotary direction to that of the rod 1 which is supplied to the melting zone 3. The additional rod 5 can be supplied advantageously to the melting zone 3 with the same velocity as that at which the rod 1 is supplied thereto.

As shown in FIG. 4, after mutually engaging the additional rod or rods 5 with the melting zone 3, the resolidifying rod portion 6 can be laterally displaced several times in opposite directions. The amplitude of the lateral displacement can be about half the radius of the rod 1 supplied to the melting zone 3. The holder 9 of the resolidifying monocrystalline rod portion 6 can also be set in rotation, however, about an axis eccentrically parallel to the center axis of the rod 1. The amplitude of the eccentric displacement can thereby advantageously be about half the radius of the rod 1. Due to the linear or eccentric displacement of the resolidifying rod portion 6, the melting zone 3 is well intermixed, resulting in a greater uniformity of the specific resistance distribution over the rod cross section. The thorough intermixing of the melting zone 3 is promoted further by placing all the rods 1, 5 supplied to the melting zone 3 in rotation. It is apparent that the advantages of the method of my invention i.e. uniform specific resistance distribution and uniform doping, are afforded when the resolidifying rod portion, after the lateral displacement, is held fast in the eccentric position and the rods supplied to the melting zone are periodically reciprocated about the center position thereof or are subjected to an eccentric movement about the center position thereof. The rods 1, 5 supplied to the melt 3 can have the same or different dimensions. Rods with different dimensions have proven to be especially advantageous when the area within circular profile or periphery of the resolidifying rod portion 6 and the area of the melting zone 3 are to be as fully utilized as possible !by the rods 1, 5 supplied to the melting zone 3. The speed, just like the dimensions of the rods 1, 5 supplied to the melting zone 3, can he the same or different. Advantageously, rods 1, 5 with smaller dimensions can be supplied at a higher displacement speed for substantially the same thermal capacity of the melting zone 3.

A heating coil 2 that is especially suitable for carrying out the method of my invention is shown in FIG. 5. The heating coil 2 has the shape of a figure eight and is so disposed that the rods 1, 5 can be passed through the openings 7 and -8, respectively, formed therein. Such a heating coil 2 has the advantage that the melting zone 3, independent of the penetrating depth of the high frequency heating current, is not only heated intensively from the outside but also simultaneously additionally the center of the melting zone 3 is heated. Consequently, for very large dimensions of the resolidifying rod portion 6 and therewith also of the melting zone 3, an exceedingly uniform temperature distribution is obtained within the melting zone 3. If three or four rods 1, 5 are to be supplied to the melting zone 3, then, advantageously, coils having three or four openings somewhat like a clover-leaf shape can be provided. It is, however, also possible to provide the individual rods 1, 5 respectively with a separate heating coil and to connect the separate heating coils to a common high frequency generator or to individual high frequency generators if necessary.

In accordance with a modified version of the method of my invention as shown in FIG. 6, the rods 1, 5 supplied to the melting zone 3 can be so disposed that they are inclined t0 the axis of the resolidifying rod portion 6, and indeed so that the center axes of the rods 1, 5 intersect at an angle in the vicinity of the melting zone 3. Such an arrangement has the advantage that sufiicient space will be provided on the upper horizontal melting chamber wall (not illustrated in the drawing) for suitable vacuum-tight passages therethrough for the rods.

It is evident that the aforedescribed and illustrated modes of the method of my invention can be suitably modified so that the resolidifying rod portion 6 is located above the heating device 2 and the melting zone 3 is passed in a direction downwardly from above by the rods 1, 5. For such modified form of the method of my invention, heating of the rods is particularly suitably effected by a flat induction coil having a plurality of spiral or helical windings.

I claim:

1. Method of crucible-free zone melting a crystalline rod which comprises rotating at least one of a pair of spaced and substantially vertically aligned end holders supporting a crystalline rod therebetween, heating the rod with an annular heating device surrounding the rod to a temperature at which a molten zone is formed in the rod dividing the rod into a supply rod portion being sup plied to the melt in the molten zone and a rod portion resolidifying from the melt, relatively displacing the end holders and the heating device in the direction of the rod axis at a given relative speed and laterally displacing the end holder for the supply rod portion out of vertical alignment with the other end holder so that the resolidifying rod portion is increased in thickness beyond the inner diameter of the annular heating device, and thereafter supplying at least one additional supply rod to the melt at a location thereof eccentric to the axis of the resolidifying rod portion, preheating the end of the additional supply rod to melting temperature in the vicinity of the molten zone to bring that end and the melt into mutual engagement.

2. Method according to claim 1 which further comprises laterally displacing the resolidifying rod portion repeatedly in opposite directions after introducing the additional supply rod into the molten zone.

3. Method according to claim 1 which further comprises rotating the resolidifying rod portion about an eccentric axis parallel to the center axis thereof after introducing the additional supply rod into the molten zone.

4. Method according to claim 1 which further comprises, after introducing the additional supply rod into the molten zone, laterally displacing the resolidifying rod portion repeatedly in opposite directions with an amplitude of about half the radius of the supply rod portion.

5. Method according to claim 1 which further comprises, after introducing the additional supply rod into the molten zone, rotating the resolidifying rod portion about an eccentric axis parallel to the center axis thereof 6 with an amplitude of about half the radius of the supply rod portion.

6. Method according to claim 1 which further comprises laterally displacing the resolidifying rod portion in opposite directions after introducing the additional supply rod into the molten zone, and thereafter periodically reciprocating the additional supply rod and supply rod portion about their center position.

7. Method according to claim '1 which further comprises laterally displacing the resolidifying rod portion in opposite directions after introducing the additional supply rod into the molten zone, and thereafter subjecting the additional supply rod and supply rod portion to an eccentric movement about their center position.

8. Method according to claim 1 which comprises rotating both the supply rod portion and the additional sup ply rod.

9. Method according to claim 1, wherein the supply rod portion and the additional supply rod have diflerent dimensions, and which comprises supplying the supply rod portion and the additional supply rod to the molten zone at different rates of speed.

10. Method according to claim 1 wherein the supply rod portion and the additional supply rod have similar dimensions, and which comprises supplying the supply rod portion and the additional supply rod to the molten zone at different speeds.

11. Method according to claim 1 wherein the supply rod portion and the additional supply rod have different dimensions, and which comprises supplying the supply rod portion and the additional supply rod to the molten zone at similar speeds.

12. Method according to claim 1 which comprises disposing at least one of the supply rod portion and the additional supply rod at an inclination toward the resolidifying rod portion so that an end of the supply rod portion and of the additional supply rod, respectively, converge at the molten zone.

References Cited UNITED STATES PATENTS 2,977,258 3/1961 Dunkle 23-301 X 3,414,388 12/1968 Keller 23301 3,470,039 9/1969 Goundry et a1. 23-301 X FOREIGN PATENTS 755,422 8/ 1956 Great Britain.

NORMAN YODKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US C X..R. 25262.9

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,539,305 Dated November 10, 1970 Inventor(s) WOLFGANG KELLER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading the German priority number should read as follows: --S 106,157 IVo/l2g-- SIGNED AND SEALED m2 197! Anew Edwndlfleuhml'i. WILLIAM E. SGHUYLER, 38..

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